Grounding electrode

ABSTRACT

A method for electric protection of a metal object, in which a long-line grounding electrode is installed in an electrolytic medium at a preset distance from the metal object to be protected and the metal object and the long-line grounding electrode are electrically connected to a current source to form a protection circuit, the metal object is polarized, while the sections of the electric connection and the geometric dimensions and/or electric parameters of the grounding electrode are so selected that the value of the current propagation constant in the protection circuit is less than or equal to 10 -3  m -1 . The grounding electrode has an extended central flexible metal conductor (18), an adhesive layer (20) providing an electric contect, and an envelope (19) of a slightly soluble current-conductive material based on a composition including a carbon-containing filler in an amount of 40-80 wt. %, a rubber-base polymer in an amount of 10-49.8 wt. %, a plasticizer in an amount of 9-10 wt. % and an insecticide in an amount of 0.2-1.0 wt. %.

FIELD OF THE INVENTION

The invention relates to electric protection of various objects, and,more specifically, to methods for electric protection of a metal object,grounding electrodes for effecting the method and compositions for thegrounding electrodes.

The invention can be used in systems of anti-corrosion cathodicprotection of elongated metal structures, for example, underground mainpipelines, as well as for electric protection of metal objects,including those of a complex shape, from external voltages.

BACKGROUND ART

Known in the art is a method for anti-corrosion cathodic protection ofan elongated metal object, in which a long-line anode in the form of acontinuous flexible steel core in an electrically conductive polymerenvelope is installed in an electrolytic medium near the surface to beprotected. In this case, the anode is disposed along the object at apre-set distance therefrom determined by the thickness of the electricinsulation plate between the anode and the surface to be protected, thenthe object and anode are connected to a polarizing current source (U.S.Pat. No. 4,487,676).

This known method however has a number of significant drawbacks. Thus,the anode is disposed in the immediate vicinity of the surface to beprotected, the distance between them is hot optimized with respect tothe electrical characteristics of the whole system. This fact, even inthe case of a plane-parallel electric field, reduces the protection andresults in nonuniform distribution of potential, especially with agedinsulation.

Furthermore, the prior art method of disposition of the protectivegrounding (anode) is associated with a danger of over-protection at thedrain point, i.e. there is a danger that the whole protection systemwill more rapidly fail.

Attempts to avoid over-protection by reducing the potential haveresulted in reduction of the protection zone, i.e. impairment of theprotection efficiency as a whole.

Known in the art is a method of cathodic protection of extended objectsby means of a flexible long-line anode, which provides an optimumdistance between the anode and the surface to be protected. The knownmethod includes installation of a long-line anode in the form of acontinuous flexible metal core encased by an electrically conductiveflexible polymer envelope in contact therewith and installed in anelectrolytic medium at a preset distance from the object, connection ofthis object and anode to current sources and polarization of the objectfrom the anode. According to this method, the anode material resistancemust be within 0.1 to 1000 ohm cm, while its longitudinal resistancemust not exceed 0.03 to 0.003 ohm m. In so doing, the anode must bearranged relative to the object to be protected so as to keep a ratio(b+D)/(a+D)<2, where a is the minimum distance between the anode and theobject to be protected, b is the maximum distance between the anode andthe object to be protected and D is the maximum linear size of theobject to be protected in the direction normal to the anode axis (U.S.Pat. No. 4,502,929).

This method is still characterized by some drawbacks hindering itsapplication. For example, the known method does not provide neededuniformity of distribution of the protective difference of potentialsalong the circumference of the insulated pipe in the process oflong-term operation. A similar negative result occurs when the pipesurface has no installation. This is due to the fact that the protectivedifference of potentials includes both the pipe potential properdetermined by the integral value of the linear density of the polarizingcurrent and the potential of the surrounding medium depending on thedifferential densities of the current flowing at each point of thevolume of the current-conductive space. Under otherwise equalconditions, the latter is substantially determined from not only theratio of the distances between the anode and the object to be protectedand the linear size of the latter but also depends on the disposition ofdamage and discontinuities in the insulation along the pipecircumference and the electrochemical properties of the surroundingground.

In many cases, with the ratio (b+D)/(a+D)<2, it is not possible toensure the required level of protection over the whole surface, e.g. asingle cross-section of a pipeline. Indeed, in the case of cathodicprotection of adjacent sections of a pipeline 1400 mm in diameter withan insulation resistance of 300 ohm m and 1000 ohm m the ratio of thedensities of the cathodic polarizable current must meet the ratio of 3:1in order to provide a uniform protective potential. In this case thepotentials of the nearest point of the ground near the pipeline with thesame departure of the anode will also meet this ratio. Assuming thatb<<D and a<<D under condition that b/a<2, it is impossible to compensatethe nonuniformity of the potentials of the ground points and, therefore,also the level of protection of adjacent sections characterized by K=3.

A similar situation is valid when a homogeneous section of a pipeline isto be protected. In this version the ground potential at the near andremote generatrix lines of the pipe remains nonuniform and this resultsin nonuniformity of distribution of the protective potential differenceover the circumference and reduces the level of protection. The limitedratio does not allow this nonuniformity to be avoided because forpipelines under the condition that b-a=D it assumes a form of a/D>0,which makes the condition of attaining uniformity of the level ofprotection indefinite.

The field of application of the method is also limited by thepredetermined therein ranges of the resistance of the anode material, aswell as that of the structure as a whole. In these ranges the anodecross section (taking no account of the flexible core) must be at least0.33-333 m² (with a diameter of 0.63-18.3 m), and this is completelyunreal. If no account is taken of the limiting values of thelongitudinal resistance of the core (0.03 to 0.0003 ohm cm) specified inthe description, its diameter should be in the range of 0.9 to 8.7 mmwhich is also unlikely taking into account the technology of manufactureand application of the anode, since this makes it less strong orflexible.

Since the attainment of a required level of protection depends ingeneral on the absolute value of the protection current and the rate ofattentuation of the current along the anode, the application of theprior art method can be inefficient in high-resistance grounds due to anincrease of the input resistance of the anode or in connection with goodcondition of the insulation coating of the object to be protected. Inthese cases, it will be impossible to obtain the required value of theprotection current due to the high contact resistance of the anode anddistribution of the required density of the protection current due to ahigh value of the constant of propagation of the current along theanode. Both these factors essentially limit the field of effectiveapplication of the known extended anodes in general and of the abovemethod in particular.

Taking into account the peculiarities of the electrochemical processestaking place in ground electrolytes, the basic requirements to thegrounding electrodes are their low rate of solubility, particularly ofthe anode, flow resistance to the current flow and uniform current yieldof the working surface of the electrode. The fulfillment of the aboverequirements provides longevity and operational efficiency of theelectrode. At the same time, conditions of cyclic transportation andassembly loads require that the electrodes should have as muchflexibility and elasticity as possible to enhance their operationalreliability.

With cathodic protection of extended structures the design of cable typeelectrodes (extended electrodes) are advantageous over pin typeelectrodes since the current yield of the extended electrodes iseffected in a plane-parallel field providing high efficiency of theprotection.

Known in the art is a grounding electrode used in cathodic protectionsystems which is made in the form of a plurality of working elements(iron-silicon anodes) distributed along a current-conducting power cableand electrically connected thereto by contact units of a special designproviding continuity of the cable and monolithic structure of theelectrode as a whole. Each working element of the electrode comprises abody with a central hole having a conical section, a continuous powercable put through the hole in the electrode body and a means for fixingthe electrode body to the cable and simultaneously providing an electriccontact therewith. The means for fixing and electric contact is made inthe form of two semi-envelopes encompassing the cable and disposed inthe hole of the electrode body. The semi-envelopes have a centralportion made of an electrically conductive material in direct contactwith the bare cable and two end conical sleeves made of an elasticdielectric material. The semi-envelopes of the fixing means aredistributed in pairs along the cable axis and form a monolithicconnection of the electrode elements using the wedge method (U.S. Pat.No. 3,326,791).

The use of iron-silicon anodes as working elements leads to electrodebrittleness and significant losses during transportation and assembly.

The contact units with conical dielectric sleeves do not providereliable enough contact due to their possible mechanical deformationduring transportation and assembly. In addition, such units do not allowprotection of the current-conductive cable against direct electriccontact with an electromagnetic medium and this results in prematuredestruction of the electrode and its failure. As a result the life ofsuch electrodes is short.

Known in the art is a flexible extended anode for cathodic protectionagainst corrosion of the internal surface of a tank made of amagnetically perceptive metal with an electrolytic medium. The anodecomprises at least one steel mainline conductor, a flexible extendedenvelope made of an electrically-conductive polymer encompassing theconductor and having an electric contact with it, and a flexibledielectric layer of a magnetic material (permanent magnet) connectedalong the anode axis with the envelope mechanically or through anadhesive layer.

The magnetic dielectric layer maintains the anode near the surface to beprotected but excludes its electric contact with the envelope. A layerof porous material (additional porous envelope) is disposed between theelectrically conductive polymer envelope of the anode (U.S. Pat. No.4,487,676).

The known anode does not allow the current distribution to be controlledwhen protecting tanks or other objects of a similar shape, i.e. withdiscretely differential quality of the surface state. The anode islimited along the length of the protection zone due to non-compensatedattenuation of the current in the monolithic electrically-conductiveenvelope and is limited by zone of protective effect (on both sides ofthe anode) due to the disposition of the anode directly on the surfaceto be protected as is necessary for the magnetic dielectric layer. Inconnection with these drawbacks, in order to guarantee a required levelof protection over the entire surface to be protected, the anode mustoperate under high current loads which results in premature wear andconsequently in a reduction of service life.

The solution which is closest to the claimed one in its technicalessence is an extended flexible electrode of an electrically-conductivepolymer composition used in systems of cathodic protection of metalobjects, e.g. pipelines. The electrode is made in the form of a band andcomprises an extended flexible metal core and an evelope of anelectrically-conductive polymer based on thermoelastoplastic materialsor plastic materials of the polyvinyl chloride type encompassing thecore in electric contact therewith and forming a working,electrochemically active surface of the electrode. The electrode may bedisposed in an additional external dielectric electrolyticallyimpermeable envelope preventing direct contact of the electrode workingsurface with the object surface (GB, A, 2,100,290).

The electrode does not have adequate reliability, especially duringassembly due to its low elasticity and frost resistance, since at atemperature of below -10° to -15° C. the envelope material startscracking. These properties of the electrode also have an adverse effecton its life. In addition, the electrode life is low due to its liabilityto biological destruction due to a low content of a filler in theenvelope material; rapid workout of the filler opens access of theelctrolyte to the core, which results in accelerated work-out, which isalso a result of a low content of plasticizer (washing out of theplasticizer and quick cracking of the electrode envelope) caused by lowmaterial capacity of the thermoelastoplastic materials and plastics usedin the envelope material.

Furthermore, the electrode design permits use of a current-conductivecore with a rated resistance of 0.5 ohm mm² /m (for comparison, theresistance of a copper core is 0.018 ohm mm² /m while that of the steelcore is 0.24 ohm mm² /m). This requires a minimum diameter of 4.5 mmwith the worst permissible resistance of 0.03 ohm/m. At the same time,the realization of the best resistance of 0.0003 ohm/m is practicallyimpossible since it is realizable with a diameter of 45 mm. At the sametime, the resistance of the material of the polymer envelope does notexceed 10 ohm m. This does not make it possible to completely utilizethe advantages of the extended electrode provided by its constantcurrent attenuation whose minimum value is 5.5 10⁻³ l/m. Under suchconditions the current load on the electrode increases, especially nearthe point of its connection and this also reduces the electrode life.

The electrically-conductive polymer compositions and electric devicesbuilt around them are well known in the art. The main components of suchcompositions are carbon-containing fillers (elementary carbon) and apolymer matrix or binder while the properties of each composition aremodified by introducing various additives depending on the designationand conditions of application of the composition (U.S. Pat. No.4,442,139).

The main requirements to the composition for grounding electrodesconsist of high electrical conductivity and low rate of solubility in anelectrolytic medium. The conditions of transportation and storage aswell as the technology of assembly of the grounding electrodes requiretheir high elasticity.

With respect to the elasticity characteristic the electrodes based onelectrically-conductive polymers are advantageous over for exampleelectrodes based on metal-oxide or iron-silicon mass used in cathodicprotection of metal structures.

However, stable combination of a high elasticity index (minimum 10%)with optimum for the given type of electrolyte (e.g. ground) indexes ofelectrical conductivity and solubility (in particular, anode) is acomplex technical problem.

An electrically-conductive composition is known having high electricalconductivity which comprises an electrically-conductive filler (metalpowder plus gas soot) and a dispersing component somewhat compatiblewith rubber, e.g. polyvinyl chloride, polystyrene, nylon, polyethyleneglycol taken in a weight ratio 40-60 and 60-40 respectively to form amixture with an elastomer binder such as natural rubber, polybutadiene,polyisoprene, ethylene-propylene rubber copolymers. The ratio of thefiller with a dispersing agent and a rubber base of the matrix in thecomposition is from 1.1:1 to 5:1 (U.S. Pat. No. 4,642,202).

The known composition has a specific resistance less than 10⁶ ohm cmwith low concentrations of the electrically-conductive filler.

However, from the point of view of its possible application forgrounding electrodes, in particular, for the anode grounders in thesystem of cathodic protection, it has a number of significant drawbacks.First, the plastics, like polyvinyl chloride and polystyrene, includedin the composition feature reversibility of deformation, which makes thecomposition inadequately elastic, particularly at low temperatures.Furthermore, the compositions based on plastic materials of thepolyvinyl chloride type have low solid matter content, i.e. low fillercontent. On the other hand, the metal powder-filler causes drasticoxidation of the polymer, particularly under the effect of the appliedcurrent, and this leads to cracking of the polymer and to loss ofelasticity.

The electrolyte penetrating through the pores and microscopic crackscauses dissolving of the metal and fast washout of the filler, whichwith a low content of the latter drastically changes the electricalcharacteristics of the composition. Thus, the metal filler in thepolymer matrix used for the known composition contributes to a rapidincrease of the specific resistance of the composition in theelectrolytic medium and stipulates its instability to anode dissolution.As a result, the insufficient vibration and frost resistance, as well asthe low flexibility of the material based on the known composition makeit practically inapplicable for the grounding electrode.

Known in the art is an electrolytic composition for coating extendedconductors which comprises in weight per cent: electrically-conductivefiller (calcined coke) 5-7%; polymer binder (ethyl lithacrylate andother acryl-latex polymers in emulsions) 5-50%; water-based solvent5-50%; surface-active additive 0-5%; thickener 0.1-10%: alcohols C₃ -C₁₂0.01-2.5%; a compound containing a bacterial anti-corrosion protectivesubstance and fungicides 0.01-2.5% (U.S. Pat. No. 4,806,272).

The composition is used in the form of an electrically-conductivecoating for cathodic protection against corrosion of steel structure ofreinforced concrete members.

However, the known composition has inadequate electrical conductivityand low resistance to anode dissolving due to weak hydrolytic stabilityof carboxyl groups, their liability to moisture absorption and thisincreases the anode dissolution. Thus, the life of the coating based onthe known comsition is low. In addition, the coating based on the knowncomposition has insufficient elasticity due to inadequate elasticity ofthe acrylates and the absence of reaction of the coke with a polymer ofthe acrylate type.

The known composition can be used only in the form of an anode layer ona cathode polymerizable structure and cannot be made in the form ofgrounding electrodes of the pin or cable type using traditional processequipment, and this limits the field of applciation of the compositionand makes it unsuitable for protection of elongated underground metalstructures.

The closest in technical essence to the claimed composition is that fora long-line flexible electrode used in systems for anti-corrosioncathodic protection of metal objects, e.g. pipelines. The compositioncomprises the following components in wt. %: an electrically-conductivefiller (gas soot or graphite) 23-55; a polymer binder (thermoplasticpolymer, polyvinyl isenfluoride and acryl resin, chlorinatedpolyethylene) 65-44.8; additives (antioxidant, calcium carbonate)0.1-5.0. The specific resistance of the composition is 0.6-29 ohm cm at23° C., its relative elongation is 10% (GB, A, 2100290).

From the point of view of possible application of the known compositionin grounding electrodes for cathodic protection of undergroundstructures, it has a number of drawbacks. In the first place, thiscomposition has low resistance to anode dissolution due to the tendencyof hydrolysis of the components such as chlorinated polyethylene,polyvinylidene fluoride used in its binding matrix, and, therefore,moisture saturation in the composition material under the effect ofground electrolytes. In the second place, the plastic materials whichare the base of its polymer matrix are not material consuming, i.e. thefiller content is limited. As an inevitable result, the filler is washedout and this drastically increases the specific resistance of thecomposition, i.e. the necessary electrical characteristics of theprotection circuit will be lost. In addition, the field of applicationof the known composition is limited due to its frost resistance. The lowfrost resistance is due to the fact that in all embodiments of thecomposition its binding matrix includes a polymer component(thermoplastic polymer, chloride or fluoride) comprising polymer linkswhich have an elevated crystallization temeprature. Thus, the strengthand electrical characteristics of the composition drasticallydeteriorate at low temperatures.

A significant drawback is also low plasticity of the composition(relative elongation is equal to 10%) and, therefore, low flexibilityand low fatigue strength of the composition material. Electrodes basedon the known composition have low resistance to cyclic strains whichalways occur during transportation and assembly.

DISCLOSURE OF THE INVENTION

The basic object of the invention is to provide a method for electricprotection of a metal object, a grounding electrode used therein and acomposition for the grounding electrode which would increase the term ofprotective effect of the grounding electrode due to a decrease of theresistance to grounding electrode current spread, uniform distributionof its potentional, lower solubility and higher frost resistance of thegrounding electrode.

This object is attained in a method for electric protection of a metalobject, in which a long-line grounding electrode comprising a centralflexible metal conductor and an envelope encompassing the centralconductor and made of slightly soluble polymer electro-conductivematerial is installed in an electrolytic medium at a preset distancefrom the metal object to be protected, the metal object to be protectedand the grounding electrode are electrically connected to a currentsource to form a protection circuit and the metal object is polarized,in that according to the invention, sections of the electric connectionto the current sources of the long-line grounding electrode and themetal object to be protected, as well as the geometric dimensions and/orelectrical parameters of the long-line grounding electrode are soselected that the value of the current propagation constant in theprotection circuit is less than or equal to 10 ⁻³ m⁻¹.

During realization of cathodic protection of a metal object at least oneadditional current source may be provided, all current sources beingconnected to the long-line grounding electrode at intervals along itslength at which a current attenuation index less than or equal to 1.5 isattained in the protection circuit.

The object of the invention is also attained due to the fact that in thegrounding electrode comprising an extended central flexible metalconductor and an envelope encompassing the central conductor and made ofslightly soluble polymeric electro-conductive material, according to theinvention, an adhesive layer ensuring an electric contact is provided onthe central conductor.

An electrically-conductive adhesive layer with electronic conductivityis arranged between the envelope and the central conductor.

It is preferable that the envelope be made of two layers and theelectrical conductivity of the layers different, and also that theenvelope has electrical parameters varying along the length of theelectrode.

It is also preferable that the adhesive layer has electrical parametersvarying along the electrode length when the central conductor ismultiple-core and surrounded by a common adhesive layer or each wire isencompassed by an adhesive layer.

It is also expedient that the flexible envelope is provided on at leasta portion of the central conductor and forms individual sections on thewhole grounding electrode, in which case the sections of the groundingelectrode free from the flexible envelope have an electricallyinsulating layer and are conjugated with the sections having theflexible envelope through a sleeve of a dielectric material surroundedby a part of the flexible envelope to form a monolithic joint; thedielectric material of the sleeve, the flexible envelope material andthe material of the electrically insulating layer are preferablyselected so that they have similar thermodynamic properties.

Each wire of the multiple-core central conductro may have sectionsprovided with an electrically insulating layer sections having noelectrically insulating layer, while the flexible envelope may encompassall sections having no electrically insulating layer, whcih areconjugated with the sections of the respective sire provided with theelectrically insulating layer through a sleeve of a dielectric materialsurrounded by a portion of the flexible envelope to form a monolithicjoint.

When the device is used for cathodic protection of a metal object, eachwire of the multiple-core central conductor may be connected to its owncurrent source belonging to an independent protection circuit.

It is desirable that at least for one wire the ratio of the length ofthe section having an electrically insulating layer to thecross-sectional area of the wire at this section varies along the lengthof the grounding electrode.

The object of the invention is also attained due to the fact that thecomposition for the grounding electrode containing a carbon-containingfiller and a binder, according to the invention, comprises arubber-based polymer as the binder and also a plasticizer and aninsecticide with the following ratio of the components in wt. %:

    ______________________________________                                        carbon-containing filler                                                                         40-80                                                      rubber-based polymer                                                                               10-49.8                                                  plasticizer         9-10                                                      insecticide        0.2-1.0                                                    ______________________________________                                    

It is advisable that the composition includes a structure stabilizer inan amount of up to 10 wt. % of the amount of the rubber-based material.

The rubber-based polymer may consist of polychloroprene or butyl rubber,or synthetic ethylene-propylene rubber while the plasticizer may consistof dibutyl phthalate or Vaseline oil or rubrax; the insecticide mayconsist of thiurams or carbamates or chlorophenols, while the structurestabilizer may consist of a mixture of magnesium chlorides and calciumchlorides or silica gel or calcined magnesia.

The proposed invention makes it possible to increase the longevity ofthe protective action of the grounding electrode, reduce the resistanceto the spread of the grounding electrode current, increase theuniformity of distribution of its potential, decrease the solubility andincrease the frost resistance of the grounding electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 shows a schematic diagram of realization of the method forelectric protection of a metal object, according to the invention;

FIG. 2 shows a schematic diagram of realization of the method forelectric protection of a reservoir, according to the invention;

FIG. 3 is the same as shown in FIG. 1 but with several current sources,according to the invention;

FIG. 4 is a cross-sectional view of the grounding electrode according tothe invention;

FIG. 5 is a cross-sectional view of the same electrode with amultiple-layer envelope, according to the invention;

FIG. 6 is a cross-sectional view of the same electrode with amultiple-core central conductor, according to the invention;

FIG. 7 is a cross-sectional view of the same electrode with amultiple-core central conductor in another embodiment according to theinvention;

FIG. 8 is a cross-sectional view of another embodiment of the electrode,according to the invention;

FIG. 9 is a cross-sectional view of the same electrode with a two-layerenvelope and a multiple-core central conductor, according to theinvention;

FIG. 10 is a longitudinal sectional view of an embodiment of a groundingelectrode with pins on the central conductor, according to theinvention;

FIG. 11 is a longitudinal sectional view of an embodiment of thegrounding electrode with an electrically insulating layer on a portionof the central conductor, according to the invention;

FIG. 12 is a longitudinal sectional view of an embodiment of thegrounding electrode with a multiple-core central conductor, according tothe invention;

FIG. 13 is a schematic diagram of effecting the method for electricprotection of a metal object, according to the invention, in which agrounding electrode with a multiple-core central conductor is used.

BEST METHOD OF CARRYING OUT THE INVENTION

The method for electric protection of a metal object is considered usingan example of protection of a pipeline 1 (FIG. 1) with the utilizationof a long-line grounding electrode 2, which is put into an electrolyticmedium 3, e.g. in the ground, at a preset distance from the pipeline 1to be protected.

The pipeline 1 through a conductor 4 and the electrode 2 through aconductor 5 are connected to a current source 6 to form a protectioncircuit, whereupon the pipeline 1 is polarized.

The source 6 has its negative terminal connected to the pipeline 1 andthe positive terminal connected to the electrode 2. As a result, duringthe operation a protection current I constantly flows, the direction ofthis current being shown by arrows 7. In so doing, the section ofconnection of the pipeline 1 and electrode 2 to the current source 6, aswell as the geometric dimensions and/or electrical parameters of theelectrode are so selected that the value of the constant α ofpropagation of the current in the protection circuit is less than orequal to 10 ⁻³ m⁻¹. This value of the current propagation constant αmust not exceed the above value since in this case the rate ofattenuation of the current in the electrode increases to such a degreethat practically excludes all advantages of current distribution andcurrent yield typical to the long-line electrode.

Depending on the above conditions, the current source 6 can be locatedon any section of the grounding electrode 2, as shown in FIG. 1 whichconditionally shows the disposition of the source 6' or 6" nearer thebeginning and end of the pipeline 1.

FIG. 2 shows a diagram of effecting the method for electric protectionof a reservoir 8 having a roof 9 which is made of dielectric materialand carries a control unit 10 connected to the body of the reservoir 8through a conductor 11 and to a current source 13 through wires 12. Thebody of the current source 13 is in turn also grounded by means of anelectrode 14. The reservoir 8 is surrounded by a long-line groundingelectrode 15 electrically connected to the body of the reservoir 8.

In case of breakdown of the insulation and appearance of a voltage onthe body of the unit 10, this voltage through the wires 12 and the bodyof the reservoir 8 is applied to the protective grounding of thelong-line electrode 15 through which the protection current 7 flowsthrough ground 16 to the electrode 14 of the working grounding of thesource 13 and the protection circuit is closed at the source 13.

FIG. 3 shows an embodiment of effecting the method for cathodicprotection of the pipeline 1 with two current sources 6 and 17, whichare electrically connected both to the pipeline 1 and to the groundingelectrode 2 in a manner similar to the conenction of the source 6 in thecircuit shown in FIG. 1. The direction of flow of the protectioncurrents I₁ and I₂ (FIG. 3) from the sources 6 and 17 is shown by arrows7. In this case, the most efficient version of the cathodic protectiondepends on the correct selection of the distance L between the sources 6and 17, which must be such as to provide a needed index of the currentattenuation in the protection circuit, i.e. the product α L less than orequal to 1.5. If the current attenuation index exceeds 1.5, the rate ofcurrent attenuation in the protection circuit increases to such a degreethat the electrode stops performing its protective functions over thewhole section of length L.

The continuous flexible extended anode is disposed at a constantdistance from the surface to be protected to form a plane-parallel fieldof the cathode current and additional limitations are introduced whichpractically level out the difference of potentials of theelectrically-conductive medium, e.g. ground, disposed around thepipeline to be protected.

It has been found in practice that under the conditions of theplane-parallel field of the current appearing with cathodic protectionusing a flexible extended anode, such a limiting condition is therelationship a>ξD [1], where a is the minimum distance between the anodeand the object to be protected, D is the maximum size of the object, ξis an empirical correlation coefficient. The observance of thisrelationship practically eliminates the nonunformity of the protectivedifference of the potentials of the structure resulting from theshielding effect.

To increase the strength and flexibility of the long-line electrode 2and to expand the field of its utilization when the transient and inputresistances are increased, a limiting ratio is introduced for theoperation of selection of the electrode and its connection through thecurrent source 6 to the pipeline to be protected.

    α.sub.1 ≦10α.sub.2                      [ 2]

where α₁, α₂ are the current propagation constants between the points ofconnection of an anode grounding 25 and an object 23 to be protected,respectively.

Satisfying the relationships [1], [2], e.g. by among other ways, layingtwo anode groundings connected to the current source 6, the rates of thecurrent attenuation along the grounding and the object 1 to be protectedare made close, thus increasing the level of efficient protection andexpanding the field of use of the grounding in high-resistance groundsdue to maximum utilization of the properties of the long-line electrode2, taking into account the current, relating to reducing the inputresistance in the protection circuit by increasing the current flowinterval while preserving allowable loss of its density due toattenuation.

As an example, let consideration be given to serveral embodiments ofcathodic protection of a section of a pipeline 320 mm in diameter with abranch of a complex configuration of a total length of 15.5 km being inoperation for 15 years and having a corrosion potential of 0.4 V m.s.e.(eith an average specific resistance of the ground equal to 30 or 100ohm m). To provide the required level of protection use was made of twokinds of connection of protective systems compensating the phenomena ofinterference and shielding with two and four current sources. Accordingto the basic methods of calculation, such sources must have maximumoutput power of 300 W. They must be equipped with concentrated anodegroundings disposed, respectively 150 or 100 m from the pipeline andconsisting, respectively, of 28-100 or 56-200 electrodes. To provide therequired operating modes of the sources, 250 or 80 W of electric energyis required respectively per year.

Various embodiments may be used in the case of using the circuits forconnection of protection systems with a long-line grounding electrode 2.Consideration will be given to the following embodiments: (1) the knownmethod of connection while fulfilling the ratio (b+D)/(a+D)<2, where bis the minimum distance between the anode and the object to beprotected; (2) the same, equivalent to the condition of α_(a) <11α_(o) ;(3) the same, equivalent to the condition of a<4.5D; (4) with observanceof the ratio (b+D)/(a+D)=3; (5) with observance of-the ratio(b+D)/(a+D)<3; (6) with observance of the relationship α_(a) =10α_(o) ;(7) with observance of the relationship α_(a) <10α_(o) ; (8) withobservance of the relationship a=5D; (9) with observance of therelationship a<6D.

The main working characteristics of the above-discussed circuits ofconnection of protective systems with different anode groundings toprovide an adequate level of protective potentials are given in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                                   Versions of cathodic                                                          protection circuits                                                           with two                                                                              with four                           Versions of cathodic protec-      sources with                                                                          sources with                        tion circuits with long-line      concentrated                                                                          concentrated                        anode groundings                  anode   anode                  System parameters                                                                          1   2   3   4   5   6  7  8   9   groundings                                                                            groundings             __________________________________________________________________________    1            2   3   4   5   6   7  8  9   10  11      12                     Specific resistance                                                                        30  100 30  30  30  30 30 100 100 30  100 30  100                of ground, ohm m                                                              Number of sources, units                                                                   100 60  30  10  10  4  4  8   8   2   2   4   4                  Composition                                                                          Number of                                                                           --  --  --  --  --  -- -- --  --  28  100 56  200                of anode                                                                             electrodes                                                             grounding                                                                            units                                                                         Cable 17.65                                                                             15.5                                                                              15.5                                                                              15.5                                                                              15.5                                                                              15.5                                                                             15.5                                                                             15.5                                                                              15.5                                                                              --  --  --  --                        length, km                                                             Length of connecting                                                                       200 120 120 20  40  8  8  8   16  300 300 400 400                cable, m                                                                      Annual consumption                                                                         0.03                                                                              0.054                                                                             0.03                                                                              0.33                                                                              0.33                                                                              0.4                                                                              0.2                                                                              0.26                                                                              0.26                                                                              0.25                                                                              0.25                                                                              0.08                                                                              0.08               of electric energy, kW                                                        __________________________________________________________________________

As seen from Table 1, the best results, as compared with the prior artmethod, are obtained using the embodiments according to the proposedmethod, i.e. with a long-line anode grounding characterized by therelationships: ##EQU1##

Therefore, the enhancement of the level of protection of objects andexpansion of the field of utilization of the method are attained byusing its technical advantages consisting of an increase in theuniformity of distribution of the protective potential and higherefficiency, as well as a reduction of the input resistance of thegrounding electrode due to the optimum distance between the electrodeand the object to be protected and the electrical characteristics of thegrounding.

The grounding electrode used in the above-described method for electricprotection of metal objects comprises an extended central flexible metalconductor 18 (FIG. 4) and an envelope 19 encompassing the conductor 18and made of a flexible slightly soluble polymer current-conductivematerial. An adhesive layer 20 providing an electric contact between theenvelope 19 and the conductor 18 is applied onto the conductor 18.

The adhesive layer 20 is electrically conductive, made, for example, ofan electrically-conductive enamel or an electrically-conductiveadhesive; the adhesive layer 20 seals the conductor 18 and the contactjoint between the conductor 18 and the envelope 19.

The envelope 19 (FIG. 5) is made two-layer and different electricalconduction of the layers 21 and 22 is provided. The envelope 19 hasvarying electrical parameters along the length of the electrode. This isattained by proper selection of the concentration of thecarbon-containing filler in the composition from which the envelope 19is made; this permits the distribution of the protection current to becontrolled, thus ensuring the differential density of the protectioncurrent as necessary for different sections of the object to beprotected.

The adhesive layer 20 along the electrode can also have varyingelectrical parameters, which is attained due to the variableconcentration of the electrically-conductive filler of the layer andenables the electrical characteristics of the electrode to becontrolled.

FIG. 6 shows an embodiment of the central conductor 18' made as amultiple-core cable, while the adhesive layer 20 surrounds the wholeconductor 18, in which case the envelope 19 is made as single-layer, asshown in FIG. 6, or two-layer, as shown in FIG. 7.

The multiple-core conductors 18 may be made differently. In FIG. 8 thecentral conductor 18 with an adhesive layer 20 is surrounded by aplurality of wires 23, each of which is encompassed by its own adhesivelayer 24. The plurality of wires 23 are in turn surrounded by a flexibleenvelope 19.

FIG. 9 shows an embodiment of the electrode, in which the centralconductor 18" comprises several wires 25, each of which is encompassedby its own adhesive layer 24. The central conductor 18" is surrounded byan envelope 19 made of two layers 21 and 22 each having differentelectrical conductivity properties (as in the embodiment described withreference to FIG. 5).

Such embodiments of the electrode make it possible to use it as aworking member of the grounding whereby a reliable contact is ensuredbetween the electrode and the current-carrying main conductor (on theinternal surface), isolation of the main conductor from the ambientmedium and uniform flow of the anode current along the whole length ofthe grounding taking into account the variable conduction of theenvelope along its length

The above-described construction ensures the following properties of thegrounding:

drastically reduces the number of contact units and eliminates theircontact with the ambient medium which enhances the reliability of theconstruction;

considerable reduces the resistance of the grounding in high-resistanceground, since it consists of a linear long-line electrode with currentleakage;

stabilizes the resistance of the grounding in time since it reduces theelectrodynamic removal of moisture due to reduction of the anode densityof the current at each point of the surface of the grounding electrode;

ensures uniformtiy of distribution of the protection current andpotential along the object to be protected due to variablydifferentiated conduction of the electrically-conductive electrodeenvelope.

In order to provide an electric contact of the central conductor 18 withthe envelope 19 when the adhesive layer is a dielectric, the centralconductor 18 has a plurality of pins 26 (FIG. 10) which penetrate intothe envelope 19 through the adhesive layer 20'. The flexible envelope 19is provided on only a portion of the length of the electrode. In theembodiment illustrated in FIG. 11, the conductor 18 has three successivesections consisting of end sections 27 and central section 28. Theflexible envelope 19 of a conductive polymeric material surrounds bothend sections 27 and central section 28. Around sections 27, theconductive polymeric envelope 19 is radially separated from the sections27 of conductor 18 by a sleeve 30 of dielectric material, e.g. ofchlorosulphonated polyethylene or a copolymer of a butadiene andstyrene-lithel styrene. The sleeve 30 forms a monolithic joint with theenvelope 19 surrounding it.

The sleeve 30, envelope 19 and electrically insulating layer 29 are madeof materials which are selected so that they have thermodynamicsimilarity. For example, this is the following combination ofmaterials: 1) the envelope 19--cis-1,4-polyisoprene rubber with acarbon-containing filler, sleeve 30--a copolymer of butadiene andstyrene, insulating layer 29--polybutadiene; 2) the envelope13--polychloroprene, sleeve 30--chlorosulphated polyethylene, insulatinglayer 29--a copolymer of butadiene and nitryl-acrylic acid.

To increase the operating life of the anode grounding, a presetalternation of the density of the leakage current of individual sectionsis provided by using anode grounding of several similarly made groundingelectrodes 31, 32 and 33 (FIG. 12).

These electrode 31-33 have the structure shown in FIG. 11, but thelength of sections 27 and 28 in each electrode 31-33 (FIG. 12) isdifferent. Furthermore, an additional envelope 19' is applied onsections when section 28 of any of electrodes 31-33 in the groundingappears. Arrows 34, 35 and 36 conditionally show the protection currentof different sections of the anode grounding.

The long-line electrode having sections of the central conductor 18 withan electrically insulating dielectric layer 29 consists electrically ofsingle current-conductive elements connected in series and characterizedby the longitudinal resistance of the conductor and transient resistanceof the current-conductive envelope 19. These two parameters control thecurrent distribution along the electrode and differentiation of thecurrent yield of each element, which are determined by the ratio of theabove resistances. Under condition of a constant specific resistance ofthe composition used for the current-conductive envelope 19 of theelectrode, the possibility of controlling the electrode characteristicsis attained due to the variability of the ratio of the length of thecross section of the conductor 18 in the dielectric layer 29. Forexample, if it is necessary to preserve the initial characteristics whenthe length of the section of the conductor with a dielectric layer 29 isreduced, the cross section of the conductor is reduced proportionally,or, which is the same, the diameter. If it is necessary to increase thecurrent yield on any local grounding element without changing itslength, it is necessary to increase the cross section of the conductor18 on the corresponding section with a dielectric layer 29.

The anode grounding of such a structure operates as follows.

The long-line type anode grounding with discretely distributedelectrical characteristics is disposed along the object to be protected.When the "minus" terminal of the current source 6 (FIG. 13) is connectedto this object 1 and the "plus" terminal is connected to the groundingelectrode, a protection current starts flowing between them. Thiscurrent produces a plane-parallel field 90-95% closed within theinterelectrode gap. The electric current flowing from the source 6spreads along the conductor 18, in which the sections 27 with anelectrically insulating layer 29 of the envelope 19 prevent its leakageto the ambient medium. At the same time, when the current reaches thecurrent-conductive sections 28 of the envelope 19, it can flow throughthe ambient medium to the nearby object 1 to be protected with atransverse gradient of potentials. Flowing into the object 1, thecurrent protects it from corrosive destruction, creating a requiredlevel of protective potential at the "object-medium" interface. Suchpropagation of current along the grounding electrode is determined bythe "long line" law, i.e. electrical characteristics: the inputresistance and the current propagation constant of the grounding itself.This allows such a ratio of dimensions of elements of thecurrent-conductive sections 28 of the envelope 19 and the distancebetween them to be discretely selected that the electricalcharacteristics of the grounding become equal to or less than thesimilar characteristics of the object 1 to be protected. In this case,optimum ondictions of current distribution in the plane-parallel fieldof protection current are attained and this increases the protectionefficiency and thus the operating life of the anode grounding underother equal conditions. The operational reliability of the grounding isincreased due to the effect of the sleeves 30 preventing prematureestablishment of a direct electric contact between the current-carryingconductor 18 and the ambient medium.

The necessity for such control of the current yield of the anodegrounding is especially pressing in case of protection of a large numberof objects, e.g. two parallel pipelines 1 and 1' having very differenttransient resistances where a preset alternation of protection currentleakage densities is needed. When the protection current only flowsthrough elements 37 of the grounding electrode, a current i_(a) flowsfrom each element to the pipeline 1 and a current i_(a) ' flows to thepipeline 1'. To provide a required level of protection, i.e. aneffective potential φ, for each pipeline 1, 1', it is necessary toprovide common potential diagrams φ₁ and φ₂ directly proportional to thetotal protection current consumption. If, in this case, the groundingconsists of two electrodes 31 and 32 with discretely distributedcurrent-conductive sections 28 of the envelope 19, currents i_(a) andi_(b) flow from these sections 28 selectively.

In this case, the currents i_(b) provide an effective potentialφ(potential diagram φ₂) on the pipeline 1' and the conditions ofprotection of the pipeline 1 remain unchanged. A comparison of thepotential diagrams φ₂ ' and φ₂ shows that the protection currentconsumption in the case of anode grounding with electrodes 31 and 32 ismuch lower and, therefore, its service life is accordingly higher underotherwise equal conditions.

The composition for the grounding electrodes includes acarbon-containing filler, a rubber-base polymer, a plasticizer and aninsecticide. The components are taken in the following proportion, wt.%:

    ______________________________________                                        carbon-containing filler                                                                         40-80                                                      rubber-base polymer                                                                                10-49.8                                                  plasticizer         9-10                                                      insecticide        0.2-1.9                                                    ______________________________________                                    

The carbon-containing filler can for example be gas soot or finelydispersed carbon-graphite dust. Such a filler provides an electronmechanism of the first kind of current yield from the metalcurrent-carrying core of the electrode to the electrode body. In sodoing, the carbon-containing filler itself has good conductivity equalto approximately 9-35 ohm m and low anode solubility which makes itpossible to considerably reduce the anode solubility of the wholecomposition of the anode grounding containing this filler in an amountof 40-80 wt. %.

The composition uses polychloroprene or butyl rubber or syntheticethylene-propylene rubber as the rubber-base polymer, and butylphtalateor Vaseline oil or rubrax as the plasticizer.

The addition of a corresponding amount (10-49.8 wt. %) of rubber-basepolymer, any of the aforementioned, to the composition, where it is inthe proposed ratio with the carbon-containing filler, provides for highelasticity (at least 30%) in combination with low specific resistancewhich, taking into account the requirements for cathodic protectionsystems, must be up to 40-50 ohm m. The elasticity, as well as low anodesolubility (0.24-0.48 kg/A year) are provided by using a plasticizer inthe composition, while an enhanced service life, especially innon-sterile electrolytic media, e.g. ground, is ensured by introducingan insecticide, such as thiurams or carbamates or chlorophenols.

A change of the proportion of plasticizer and insecticide beyond theproposed limits impairs the basic properties of the composition. Anincreased content of the rubber-base polymer, or, which is the same, adecreased content of the carbon-containing filler, making it possible toreduce content of the plasticizer results in a drastic increase of thespecific resistance of the composition. A reduced content of said binderor an increased content of the carbon-containing filler reduces theelasticity of the composition. To maintain it at the required leve, thecontent of the plasticizer has to be increased and this also causes thespecific volumetric resistance of the composition to substantiallyincrease.

Reduction of the content of the insecticide to a value less than 0.2%deprives the composition of antibacterial stability, while its increaseto a value higher than 1.0% makes the composition toxic which isforbidden by sanitary regulations.

Thus, the proposed interrelated proportion of the components of thecomposition provides for three basic quantitative parameters:

    ______________________________________                                        anode solubility   not higher than                                                               0.24-0.48 kg/A year                                        specific resistance                                                                              not higher than                                                               40-50 ohm m                                                elasticity         minimum 30%                                                ______________________________________                                    

The composition for the grounding electrode is prepared as follows.

Using rolls at a temperature of 40-90° C. a rubber-base polymer isprepared, which is then supplemented with a carbon-containing filler, aplasticizer and an insecticide. At the beginning of the process ofmixing the binder is plasticized for from one to five minutes. Then,after six to nine minutes, the plasticizer and insecticide are added.The carbon-containing filler is added during the 10th to 18th minute.The mixing process is completed at the 19th to 21st minute. Thevulcanization is effected in an electrical press at a temperature of140°-160° C.

Mixtures were prepared having different amounts and types of components.The data are tabulated in Table 2. The results of a study of the effectof the amount of each component on the composition properties are givenin Table 3.

                                      TABLE 2                                     __________________________________________________________________________               Content of components, wt. %                                       Carbon-    Rubber-base polymer                                                Item No.                                                                            contain-       Ethylene-                                                of compo-                                                                           ing  Polychloro-                                                                         Butyl                                                                             propylene                                                                           Dibutyl-                                                                           Plasticizer                                   sition                                                                              filler                                                                             prene rubber                                                                            rubber                                                                              phthalate                                                                          Vaseline oil                                                                        Rubrax                                                                            Insecticide                         1     2    3     4   5     6    7     8   9                                   __________________________________________________________________________     1    40   49.8  --  --    10.0 --    --  0.2                                  2    40   49.8  --  --    9.5  --    --  0.7                                  3    40   49.8  --  --    9.2  --    --  1.0                                  4    40   --    49.8                                                                              --    --   10.0  --  0.2                                  5    40   --    49.8                                                                              --    --   9.5   --  0.7                                  6    40   --    49.8                                                                              --    --   9.2   --  1.0                                  7    40   --    --  49.8  --   --    10.0                                                                              0.2                                  8    40   --    --  49.8  --   --    9.5 0.7                                  9    40   --    --  49.8  --   --    9.2 1.0                                 10    60   29.8  --  --    10.0 --    --  0.2                                 11    60   29.8  --  --    9.5  --    --  0.7                                 12    60   29.8  --  --    9.2  --    --  1.0                                 13    60   --    29.8                                                                              --    --   10.0  --  0.2                                 14    60   --    29.8                                                                              --    --   9.5   --  0.7                                 15    60   --    29.8                                                                              --    --   9.2   --  1.0                                 16    60   --    --  29.8  --   --    10.0                                                                              0.2                                 17    60   --    --  29.8  --   --    9.5 0.7                                 18    60   --    --  29.8  --   --    9.2 1.0                                 19    80   10.0  --  --    9.8  --    --  0.2                                 20    80   10.0  --  --    9.4  --    --  0.6                                 21    80   10.0  --  --    9.0  --    --  1.0                                 22    80   --    10.0                                                                              --    --   9.8   --  0.2                                 23    80   --    10.0                                                                              --    --   9.4   --  0.6                                 24    80   --    10.0                                                                              --    --   9.0   --  1.0                                 25    80   --    --  10.0  --   --    9.8 0.2                                 26    80   --    --  10.0  --   --    9.4 0.6                                 27    80   --    --  10.0  --   --    9.0 1.0                                 28    60   29.7  --  --    10.0 --    --  0.3                                 29    60   30.0  --  --    9.2  --    --  0.8                                 30    60   --    29.7                                                                              --    --   10.0  --  0.3                                 31    60   --    30.0                                                                              --    --   9.2   --  0.8                                 32    60   --    --  29.7  --   --    10.0                                                                              0.3                                 33    60   --    --  30.0  --   --    9.2 0.8                                 __________________________________________________________________________

Thiurams are used as the insecticide in compositions Nos. 1-3, 10-12,19-21, 28, 29, carbamates are used in compositions Nos. 4-6, 13-15,22-24, 30, 31, while chlorophenols are used in the remainingcompositions.

                  TABLE 3                                                         ______________________________________                                                          Specific                                                           Anode solu-                                                                              resistance                                                  Composi-                                                                             bility of  of compo-                                                   tion   composition                                                                              sition,    Elasti-                                                                             Antibacterial                              No.    kg/A year  ohm m      city  resistance                                 ______________________________________                                         1     0.15       50         41    resistant                                   2     0.17       50         38    resistant                                   3     0.18       48         32    resistant                                   4     0.19       50         41    resistant                                   5     0.21       50         35    resistant                                   6     0.23       45         30    resistant                                   7     0.24       50         40    resistant                                   8     0.26       48         31    resistant                                   9     0.28       40         30    resistant                                  10     0.25       50         42    resistant                                  11     0.26       48         40    resistant                                  12     0.27       45         35    resistant                                  13     0.23       48         42    resistant                                  14     0.26       45         38    resistant                                  15     0.29       40         34    resistant                                  16     0.27       48         35    resistant                                  17     0.28       46         34    resistant                                  18     0.29       39         32    resistant                                  19     0.28       46         36    resistant                                  20     0.31       38         34    resistant                                  21     0.35       34         31    resistant                                  22     0.31       44         36    resistant                                  23     0.36       36         32    resistant                                  24     0.41       32         30    resistant                                  25     0.35       42         34    resistant                                  26     0.42       30         31    resistant                                  27     0.48       28         30    resistant                                  28     0.25       48         38    resistant                                  29     0.25       48         41    resistant                                  30     0.25       50         36    resistant                                  31     0.24       49         40    resistant                                  32     0.25       49         38    resistant                                  33     0.24       50         40    resistant                                  ______________________________________                                    

It is evident from Table 3 that the rate of anode solubility of thegroundings made of the proposed compositions is several times less thanthe known one. Therefore, the use of dibutyl phthalate and rubber-basepolymer of the chloroprene type as a plasticizer in the proposedproportions makes it possible to reduce the average rate of dissolvingby a factor of 1.8 to 2.9, i.e. to accordingly increase the service lifeof the grounding electrodes made of these compositions in the sameproportion. Similar use of a rubber-base polymer of the butyl rubbertype and a plasticizer of the Vaseline oil type makes it possible toreduce the average rate of dissolving by a factor of 1.6 to 2.5, whilethe use of a rubber-base polymer of the synthetic ethylene-propylenetype and a plasticizer of the rubrax type reduces the same by a factorof 1.4 to 2.2, i.e. as a whole on the average by a factor of two.

The anode solubility of practically all compositions is less than thatof the prior art composition and this makes it possible to increase thelife of the anode grounding electrodes made of these compositions by10-15 years.

Introduction of the insecticide into the composition makes it resistantto bacterial destruction when the insecticide is added in an amount ofminimum 0.2%.

An increase of the insecticide content above 1.0% makes the process ofpreparation of the composition toxic and the final products of thisprocess are in many cases also toxic. The necessary protection measurescomplicate the technology of making the composition, while the practicalutilization of toxic articles is prohibited by sanitary regulations.

Examples of compositions with different insecticides, i.e. thiurams,carbamates and chlorophenols are given in Table 4, in which the othercomponents are taken in proportions corresponding to the compositionnumber given in columns 3-8 of Table 2.

                  TABLE 4                                                         ______________________________________                                        Composi-                                                                      tion No. Thiurams    Carbamates                                                                              Chlorophenols                                  ______________________________________                                         1       0.2         --        --                                              2       0.7         --        --                                              3       1.0         --        --                                              4       --          0.2       --                                              5       --          0.7       --                                              6       --          1.0       --                                              7       --          --        0.2                                             8       --          --        0.7                                             9       --          --        1.0                                            10       0.2         --        --                                             11       0.7         --        --                                             12       1.0         --        --                                             13       --          0.2       --                                             14       --          0.7       --                                             15       --          1.0       --                                             16       --          --        0.2                                            17       --          --        0.7                                            18       --          --        1.0                                            19       0.2         --        --                                             20       0.6         --                                                       21       1.0         --                                                       22       --          0.2       --                                             23       --          0.6       --                                             24       --          1.0       --                                             25       --          --        0.2                                            26       --          --        0.6                                            27       --          --        1.0                                            28       0.3         --        --                                             29       0.8         --        --                                             30       --          0.3       --                                             31       --          0.8       --                                             32       --          --        0.3                                            33       --          --        0.8                                            ______________________________________                                    

To improve the composition, it is provided with a structure stabilizerin an amount of up to 10 wt. % of the rubber-base polymer. If the amountof the structure stabilizer exceeds 10 wt. %, the composition does notsatisfy the permissible lower elasticity limit, and therefore themechanical properties of the electrodes deteriorate and their servicelife is reduced.

A mixture of chlorides of magnesium and calcium or silica gel orcalcined magnesia is used as the structure stabilizer. Examples ofcompositions with a structure stabilizer, whose amount is selectedrelative to one of said rubber-base polymers, are summarized in Table 5.The other components are taken in amounts given in Table 2 for therespective composition number.

                  TABLE 5                                                         ______________________________________                                                         Structure stabilizer                                         Item                   mixture                                                No.                    of chlo-                                               of   Rubber-base polymer                                                                             rides of                                               com- poly-           ethylene-                                                                             magnesi-    calcined                             posi-                                                                              chloro- buty    propylene                                                                             um and silica                                                                             mag-                                 tion prene   rubber  rubber  calcium                                                                              gel  nesia                                ______________________________________                                         1   45.27   --      --      4.52   --   --                                    2   45.27   --      --      4.52   --   --                                    3   45.27   --      --      4.52   --   --                                    4   --      45.27   --      --     4.52 --                                    5   --      45.27   --      --     4.52 --                                    6   --      45.27   --      --     4.52 --                                    7   --      --      45.27   --     --   4.52                                  8   --      --      45.27   --     --   4.52                                  9   --      --      45.27   --     --   4.52                                 10   27.091  --      --      2.71   --   --                                   11   27.091  --      --      2.71   --   --                                   12   27.091  --      --      2.71   --   --                                   13   --      27.09   --      --     2.71 --                                   14   --      27.09   --      --     2.71 --                                   15   --      27.09   --      --     2.71 --                                   16   --      --      27.1    --     --   2.71                                 17   --      --      27.1    --     --   2.71                                 18   --      --      27.1    --     --   2.71                                 19   10.0    --      --      1.0    --   --                                   20   10.0    --      --      1.0    --   --                                   21   10.0    --      --      1.0    --   --                                   22   --      10.0    --      --     1.0  --                                   23   --      10.0    --      --     1.0  --                                   24   --      10.0    --      --     1.0  --                                   25   --      --      10.0    --     --   1.0                                  26   --      --      10.0    --     --   1.0                                  27   --      --      10.0    --     --   1.0                                  28   27.0    --      --      2.7    --   --                                   29   27.0    --      --      2.7    --   --                                   30   --      27.0    --      --     2.7  --                                   31   --      27.0    --      --     2.7  --                                   32   --      --      27.0    --     --   2.7                                  33   --      --      27.0    --     --   2.7                                  ______________________________________                                    

In compositions Nos. 19-27 the carbon containing filler is taken in anamount of 79 wt. %.

Table 6 presents some physical characteristics of grounding electrodeswith compositions used containing the polymers given in Tables 2-5.

                  TABLE 6                                                         ______________________________________                                              Anode                                                                         solubi-   Speci-                                                        Com-  lity of   fic            Operating                                      posi- composi-  resis-         stability                                                                             Anti-bac-                              tion  tion kg/A tance,  Elasti-                                                                              of resis-                                                                             terial                                 No.   year      ohm m   city, %                                                                              tance, %                                                                              resistance                             ______________________________________                                         1    0.15      50      41     80      resistant                               2    0.17      50      38     85      resistant                               3    0.18      48      32     95      resistant                               4    0.19      50      41     80      resistant                               5    0.21      50      35     85      resistant                               6    0.23      45      30     95      resistant                               7    0.24      50      41     80      resistant                               8    0.26      48      31     85      resistant                               9    0.28      40      30     95      resistant                              10    0.25      50      42     80      resistant                              11    0.26      48      40     85      resistant                              12    0.27      45      35     95      resistant                              13    0.23      48      42     80      resistant                              14    0.26      45      38     85      resistant                              15    0.29      40      34     95      resistant                              16    0.27      48      35     80      resistant                              17    0.28      46      34     85      resistant                              18    0.29      39      32     95      resistant                              19    0.28      46      36     80      resistant                              20    0.31      38      34     85      resistant                              21    0.39      34      31     95      resistant                              22    0.31      44      46     80      resistant                              23    0.36      36      32     85      resistant                              24    0.41      32      30     95      resistant                              25    0.35      42      34     80      resistant                              26    0.42      30      31     85      resistant                              27    0.48      28      30     95      resistant                              28    0.25      48      38     85      resistant                              29    0.25      48      41     95      resistant                              30    0.25      50      36     85      resistant                              31    0.24      49      40     95      resistant                              32    0.25      49      38     85      resistant                              33    0.24      50      40     95      resistant                              ______________________________________                                    

Therefore, the claimed composition for the grounding electrodes featurestechnological advantages and has high elasticity and low specificresistance, as well as high resistance to anode dissolving and againstbacterial destruction. This makes it possible to reduce the number andto increase the effective service life of such electrodes in anodegroundings on the average by 100%. This is very important since withelectrochemical protection of underground structures against corrosion,the installation and replacement of anode groundings constitute the mainpart of the building expenses.

INDUSTRIAL APPLICABILITY

The invention can be used in systems of anti-corrosion cathodicprotection of extended metal structures such as main pipelines, as wellas for electrical protection of metal objects, including objects ofcomplex shape, against external voltages.

We claim:
 1. A grounding electrode for electrically protecting a metalobject, comprising:(a) a central elongate flexible metal conductorhaving successive first and second axial sections, (b) an envelope, madeof a flexible electrically conductive polymeric material, surrounding aportion of the central conductor, (c) an insulating layer surroundingpart of the central conductor, (d) a layer of conductive adhesive, and(e) a sleeve of dielectric material; the conductive polymeric envelopebeing positioned to surround said first and second sections of saidcentral metal conductor; the layer of conductive adhesive beingpositioned between the central conductor and the conductive polymericenvelope in said first section of the grounding electrode; and saidsleeve of dielectric material being positioned around said insulatinglayer within the conductive polymeric envelope of said second section ofthe central conductor and forming a monolithic joint with said envelope.2. A grounding electrode according to claim 1, characterized in that theconductive polymeric envelope consists of two layers and the elctricalconductivity of the layers is selected to be different.
 3. A groundingelectrode according to claim 2, characterized in that the conductivepolymeric envelope has electrical parameters varying along the length ofthe electrode.
 4. A grounding electrode according to claim 3,characterized in that the conductive adhesive layer has electricalparameters varying along the length of the electrode.
 5. A groundingelectrode according to claim 4, characterized in that the centralconductor is a multiple-core conductor surrounded by a common adhesivelayer.
 6. A grounding electrode according to claim 1, characterized inthat the conductive polymeric envelope has electrical parameters varyingalong the length of the electrode.
 7. A grounding electrode according toclaim 1, characterized in that the conductive adhesive layer haselectrical parameters varying along the length of the electrode.
 8. Agrounding electrode according to claim 1, characterized in that thecentral conductor is a multiple-core conductor surrounded by acommon-adhesive layer.
 9. A grounding electrode according to claim 1,characterized in that the central conductor is a multiple-core conductorcomprising a plurality of wires, and a conductive adhesive layerencompasses each of the wires of the multiple-core conductor.
 10. Agrounding electrode according to claim 1, wherein said central conductoris a multiple core conductor comprising a plurality of wires.
 11. Agrounding electrode according to claim 10, characterized in that for atleast one wire the ratio of the length of the section provided with anelectrically insulating layer to the cross-sectional area of the wire inthis section is so selected that the said ratio varies along the lengthof the grounding electrode.